Bd. VI: 4) THE ECHINOIDEA. 33 



This change in size of the tubercles above the ambitus looks not so abrupt as in 

 A. Dufresnii, on account of the occurrence of larger tubercles between the small 

 ones. Near the apical system the two series become irregular, as is commonly the 

 case in the species of Arbacia. The large tubercles at the ambitus occupy the 

 whole height of the compound plate, the upper and lower edge of their base being 

 more or less straight. The side of the tubercle-base looking towards the pores is 

 somewhat deeply indented. The median line of the ambulacra is covered by nu- 

 merous small miliary tubercles carrying pedicellariae. The pores are arranged in a 

 somewhat sinuating line. The epistroma is very strongly developed above and out- 

 side each pair of pores (on the abactinal side) — a conspicuous difference from Du- 

 fresnii, where it is very slightly developed (PI. XV Fig. 1 to compare with PI. XV 

 Fig. 3). Towards the peristome the pore area widens very considerably, being 

 twice as broad as the interporiferous zone, and the whole ambulacral area is at the 

 edge of the peristome twice as broad as the interambulacral area. 



The interambulacral plates carry at the ambitus each three large tubercles, some- 

 times with a small fourth tubercle at the inner edge; there are thus 6 vertical series 

 of interambulacral tubercles. The 2 — 3 plates immediately above the ambitus carry 

 each four tubercles, gradually diminishing in size from the outer towards the inner 

 one. The third of these tubercles continues until the 4 — 5th plate from above, the 

 second continues up to the apical system, only on the uppermost plate it has not yet 

 appeared. The outer series likewise continues to the apical system, without any inter- 

 ruption; the tubercles gradually diminish in size, only exceptionally a single tubercle 

 may occur, which exceeds the one immediately below somewhat in size. — The 

 space between the primary tubercles is on the actinal side occupied by small miliary 

 tubercles, carrying pedicellariae; on the abactinal side a fairly well developed epi- 

 stroma occupies the part of the plates not covered by the tubercles, together with 

 the white miliary tubercles, carrying pedicellarise, scattered between the elevations of 

 the epistroma. — The inner edge of the plates is finely granular. — The strong 

 development of the abactinal interambulacral tubercles (and spines), leaving scarcely 

 any naked median space, forms a most conspicuous difference from Dufresnii, which 

 latter species has always a very large naked space in the interambulacra on the ab- 

 actinal side. (PI. V Figs. 2 — 3 to compare with PI. V Figs. 4 — 7, 9—12.) 



The apical system upon the whole bears a considerable resemblance to that of 

 Dufresnii; the shape of the genital plates is alike, and the ocular plates are all ex- 

 cluded from the anal area as in that species (Figs. 8 — 9). The shape of the ocular 

 plates, however, differs from that in Dufresnii; the outer sides are longer and some- 

 what concave, and the whole plate is conspicuously smaller than in Dufresnii; also 

 the outer edge is somewhat different (PI. XV Fig. 11 comp. with PI. XV Fig. 9). 

 The ocular pore is, as usually in Arbacia, covered by a prominent tubercle. The 



5 — 100133. Sc/iwedische Sudpolar-Expedilion igoi — rgoj. 



